Three frequency modulated thermal neutron lifetime log

ABSTRACT

Methods are disclosed for measuring simultaneously the thermal neutron lifetime of the borehole fluid and earth formations in the vicinity of a well borehole. A harmonically intensity modulated source of fast neutrons is used to irradiate the earth formations with fast neutrons at three different intensity modulation frequencies. Intensity modulated clouds of thermal neutrons at each of the three modulation frequencies are detected by a single spaced detector and the relative phase shift of the thermal neutrons with respect to the fast neutrons is determined at each of the three modulation frequencies. These measurements are then combined to determine simultaneously the thermal neutron decay time of the borehole fluid and the surrounding earth formation media.

BACKGROUND OF THE INVENTION

This invention relates to in situ measurements of earth formationstraversed by a well borehole. In particular the invention relates to themeasurement of the thermal neutron lifetime or thermal neutron decaytime of earth formations in the vicinity of a well bore.

The techniques used in the present invention include the generation as afunction of time of a phase coherent intensity modulated cloud of fastneutrons in a well bore which results in the creation of a phasecoherent thermal neutron cloud being produced as the fast neutrons areslowed to thermal energy by the materials in the vicinity of the wellbore. Measurements of the relative phase of the thermal neutron cloudpopulation density with respect to the fast neutron cloud generated leadto the determination of parameters relating to the thermal neutron decaytime or neutron lifetime of the formations in the vicinity of the wellbore and the borehole fluid itself.

BRIEF DESCRIPTION OF THE PRIOR ART

At the present time there are two principle techniques used formeasuring, in situ, the thermal neutron decay time or thermal neutronlifetime of earth formation in the vicinity of well borehole. Theseneutron lifetime measurements have proven to be particularly valuable inevaluating earth formations in cased well boreholes. In both of thesetechniques a logging instrument which traverses the well bore uses apulsed source of high energy or fast (14 MEV) neutrons.

In the first of these measurement techniques the neutron source isrepetitively pulsed. For each fast neutron pulse, a cloud of fastneutrons is injected in a generally spherically symmetric fashion aboutthe source to the surrounding earth formations. The fast neutron cloudpasses from the well tool through the drilling mud, well bore casing,and cement between the casing and earth formations surrounding the wellbore. Each such pulse of fast neutrons has approximately a constantintensity and lasts typically for a time duration of from 20 to 30microseconds. This time lapse is generally adequate to create athermalized (or low energy) neutron population in the earth formationsand borehole. The number of thermal neutrons comprising this cloud orpopulation then decays exponentially due to the capture of thethermalized neutrons by formation and borehole elemental nuclei.

After an initial time period, (about 300 microseconds) during whichresultant gamma ray effects in the borehole, mud, and casing aresubstantially dissipated, measurements of the number of thermalizedneutrons in the vicinity of the well tool are made during two successivetime intervals and can be used to define an exponential decay curve forthe thermal neutron population either in the borehole or the earthformation surrounding the borehole. Which of these two thermal neutrondecay characteristics is being measured is not known with certainty dueto the fact that the assumption is made in this measurement techniquethat the borehole thermal neutron decay time is substantially shorterand hence dies out quicker than that of the surrounding earthformations. This assumption that the borehole component of thermalneutron decay time (or thermal neutron lifetime) is generally shorterthan the formation thermal neutron decay time or thermal neutronlifetime usually occurs where drilling fluids having a high chlorinecontent (or salt water content) are encountered. However, in boreholescontaining air, gas, fresh water or oil this relationship does notalways hold. One striking advantage of the present invention over thisprior art thermal neutron lifetime measuring technique is that noassumption is made as to the relative thermal neutron decaycharacteristic of the borehole fluid or with respect to that of theformations surrounding the borehole. Accordingly, the present inventionovercomes the aforementioned limitation of the prior art.

Measurements of the number of thermalized neutrons in the vicinity ofthe well tool during the successive time intervals following the initialtime lapse to allow for die away of borehole effects can be used todefine an exponential decay curve for the thermal neutron population ofthe earth formations in the vicinity of the borehole.

These two time intervals or time gates, for example, can be fixedbetween 400-600 microseconds following the neutron burst, and between700-900 microseconds following the neutron burst in typical earthformations, and under borehole conditions wherein a saline fluid or highchlorine content salt is present in the borehole fluid.

If neutron diffusion effects are ignored, the relationship for the decayof a thermal neutron population in a homogenous medium having a thermalneutron macroscopic capture cross-section can be expressed as:

    N.sub.2 = N.sub.1 e.sup.-.sup.ε.sup.vt             ( 1)

wherein N₁ is the number of thermal neutrons at a first point in time,t₁ ; N₂ is the number of thermal neutron at a later point in time, t₂ ;e is the Naperian logarithm base; t is the time between two measurements(t₂ -t₁); and v is the velocity of the thermal neutrons. The macroscopicthermal neutron capture cross section ε of a reservoir rock (which canbe obtained from Equation (1) is dependent upon its porosity, theformation water salinity, and the quantity and type of petroleumcontained in the pore spaces therein and thus is a valuable measurementto obtain.

When neutrons from the high energy neutron source interact with thematerials in a well bore and with surrounding earth formations, they areslowed down and lose energy. A primary agent for slowing down neutronsis hydrogen which is relatively available in water and hydrocarbon.After the fast neutrons have been slowed they are captured by formationnuclei (primarily by chlorine) and, in general, will generatecharacteristic capture gamma rays before returning to a stable state. Itis the capture gamma rays which are detected during the two differenttime intervals of this system of measuring thermal neutron decay time.The number of such gamma rays detected is proportional to the thermalneutron population in the vicinity of the well tool. Alternatively,thermal neutrons themselves can be detected during these intervals bythe use of helium 3 or boron trifluoride detectors if desired. Thus, bymeans of the two fixed time gating measurements the thermal neutronmacroscopic capture cross section ε, can be determined.

A second prior art technique for measuring thermal neutron decay time orthermal neutron lifetime uses the reciprocal of the macroscopic thermalneutron capture cross section ε which is defined in terms of τ (the timeconstant for absorption of the thermal neutrons). A relationshipanalogous to Equation (1), but defined in terms of τ is given by:

    N = N.sub.o e.sup.-.sup.t/.sup.τ                       ( 2)

where τ = 1/vε

Here N is thermal neutron density at any time t; N_(o) is the thermalneutron density at an initial time t_(o) ; e is the Naperian constant; τis the time required for the thermal neutron population to decay to 1/eof its value at t_(o).

In measuring the thermal neutron decay time using this second prior arttechnique, the logging equipment obtains counts of capture gamma raysduring two successive time intervals following the generation of thethermal neutron cloud in the vicinity of the well borehole to define theexponential decay curve. In this technique, however, the two timeintervals of the measurement are defined as a function of the τ actuallymeasured during a previous measurement cycle. The value of τ previouslymeasured is used to establish the neutron burst duration for thegeneration of the fast neutrons; the waiting interval to the opening tothe first time gate, the duration of the first time gate, the durationof the time between the time gates and the duration of the second timegate. All of these times are related to τ, as previously measured. Thistechnique is commonly referred to in the art as the "sliding gate"technique.

Both of the foregoing systems of measurement have been successfully usedto measure the decay time or lifetime of thermal neutrons as long as theborehole component of thermal neutron population dies away substantiallyfaster than the formation component of the thermal neutron population inthe vicinity of the borehole. Both of these techniques utilize a neutronburst of substantially constant intensity (a square wave neutron pulse).

BRIEF DESCRIPTION OF THE INVENTION

In the present invention a well logging tool is moved through theborehole and includes an intensity modulated fast neutron source and agamma ray detector (or, alternatively, a thermal neutron detector). Asingle such detector is used. The neutron source generates a generallyharmonically varying population of fast neutrons as a function of time.These neutrons are introduced into the media surrounding the wellborehole and result in a thermal neutron population being generated fromthe slowing down of the fast neutrons in the media and borehole itself.This cloud of thermalized neutrons itself comprises a phase coherentintensity varying neutron population whose presence is detected as afunction of time by the thermal neutron or gamma ray detector. The fastneutron source is harmonically or sinusoidally modulated at a pluralityof frequencies. The relative phase angle of the phase coherent thermalneutron population which is generated at each of the modulationfrequencies is detected by the detector. This phase angle informationcontains components due to the effect of the borehole fluid and themedia surrounding the borehole. By appropriately combining thesemeasurements made at the plurality of different modulation frequenciesaccording to predetermined relationships, the thermal neutron lifetime(or thermal neutron decay time) of the borehole fluid and the mediasurrounding the well bore may be determined.

In practice, the measurements of the relative phase angles at each ofthe modulation frequencies are made by combining counts from thedetector in a particular manner to derive the value of the tangent ofthe phase angle. From this tangent information the formation thermalneutron decay time τ or the corresponding thermal neutron macroscopiccapture cross section value ε can be established.

Novel electronic systems are provided in the downhole tool and at thesurface for producing a sequence of different frequency intensitymodulated fast neutron clouds operating at least at three differentfrequencies of operation. Synchronization (or sync) pulses are alsogenerated and these provide a means for separating the counts of gammarays representative of thermal neutrons during the portion of ameasurement cycle corresponding to measurements made at each of thedifferent frequencies of intensity modulation of the neutron source. Atthe earth's surface the signals from the downhole tool are separated andcounts are made as a function of time of the thermal neutron populationin the vicinity of the tool at each of the three frequencies. Thesecounts are used to determine the tangents of the relative phase anglesbetween the thermal neutron populations and the source of fast neutronsat each of the frequencies. A recorder is provided for making a recordof these measurements as a function of the borehole depth of the welltool. Moreover, the invention includes techniques for determining thevalue of the thermal neutron decay time τ and/or the macroscopic thermalneutron capture cross section ε of both the media surrounding theborehole and the borehole fluid.

The invention is best understood by reference to the following detaileddescription thereof, when taken in conjunction with the accompanyingdrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall block diagram illustrating schematically theapparatus of the invention in a well borehole.

FIG. 2 is a circuit diagram showing the downhole portion of the systemof the invention.

FIG. 3 is a circuit diagram showing the surface portion of the system ofthe invention.

FIGS. 4 and 4a are graphical representations showing the relationshipbetween the phase angle tangents at two different modulationfrequencies.

FIGS. 5 and 5a are graphical representations showing the relationshipbetween the phase angle tangents at two different modulationfrequencies.

FIG. 6 is a graphical representation showing the relationship betweenthermal neutron lifetime measured by phase angles measured at twodifferent pairs of frequencies of modulation.

FIG. 7 is a schematic illustration of the waveform of signals sent tothe surface from the downhole tool in the invention; and

FIG. 8 is a schematic illustration showing the measurement cycle orsequence of neutron outputs at three different frequencies of intensitymodulation of the neutron source.

FIG. 9 is a graphical illustration showing the counting periods used todetermine the relative phase shifts.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention contemplates making measurements of the intensityphase shift of a phase coherent thermal neutron population cloud withrespect to the intensity modulated fast neutron population generated bya neutron source at three different intensity modulation frequencies. Byappropriately combining the measurements of the phase shift at each ofthe frequencies the thermal neutron lifetime (or decay time) of both theborehole component of the neutron flux and the formation component ofthe neutron flux may be determined.

Referring initially to FIG. 1, a system in accordance with the conceptsof the invention is illustrated schematically. A well borehole 10traverses earth formations 11, and is lined with a tubular casing 12,which is cemented in place by cement layer 13. Suspended in the borehole10 is a well logging sonde 14 which is suspended therein by an armoredwell logging cable 15. The cable 15 passes over a sheave wheel 16 whichis electrically or mechanically linked (as indicated by the dotted line17) to a recorder 18 of the type conventionally used in well logging.The record medium 19 of the recorder 18 may thus be driven as a functionof the borehole depth of the well logging tool. The borehole 10 isfilled with a fluid 20 which may be either a salt (saline) solution orfresh water or oil. The present invention works equally well in salt orfresh water environments which is an advantage over prior art thermalneutron lifetime or decay time measuring systems which did not performwell in fresh water or oil filled boreholes.

The well logging sonde 14 used with the present invention contains aneutron generator 21, circuitry for driving the neutron generator 22,and a gamma ray detector 23. Appropriate circuitry 24 for controllingthe neutron generator 21 and for amplifying signals from the detector 23is also provided. A surface power supply 28 provides operative power forthe downhole systems via the conductors of the well logging cable 15.Signals from the downhole tool are processed by surface data processingcircuits 25 which will be described in more detail subsequently. Outputsignals from the data processing circuits 25 are supplied to therecorder 18 whose record medium 19 is driven as a function of depth aspreviously mentioned.

The gamma ray detector 23 may be a thallium doped sodium or cesiumiodide crystal which is optically coupled to a photomultiplier tube. Acontrol electronic section 24 provides signals, as will be subsequentlydescribed in more detail, to operate the neutron generator 21 and thephotomultiplier and detector 23. Radiation detected by the detector 23is presented for transmission to the surface as sharply peaked positivevoltage pulses which will be described in more detail subsequently.These pulses are presented to the cable 15 conductors for transmissionto the surface. Similarly, large amplitude sharp peaked synchronization(negative voltage) pulses are also supplied, as will be described inmore detail subsequently, for transmission to the surface, on cable 15conductors.

The neutron generator 21 is preferably of the deuterium-tritiumaccelerator type as known in the art. This type of neutron sourceaccelerates deuterium ions onto a target material which is impregnatedwith tritium. The deuterium ions are supplied from a replenisher whichcomprises a material impregnated with deuterium which is boiled off byheating. The deuterium atoms thus provided are then supplied to an ionsource comprising a region of the tube in which electric fields areprovided to ionize the atomic deuterium from the replenisher and tofocus the positive ions into a beam suitable for acceleration onto thetarget material. The target material is generally kept at a highnegative potential. The ion source is provided with an electrodestructure analogous to that of a conventional triode vacuum tube andhaving an element analogous to the control grid of such a tube. Byapplying a time varying voltage to this control grid element theresultant intensity of neutron output of the accelerator tube may bemodulated as a function of time. In this manner a harmonically orsinusoidally alternating intensity neutron output may be produced. For amore detailed description of the methods employed to produce anintensity modulated fast neutron flux as a function of time, referencemay be had to the paper entitled "The Generation of Neutron Pulses andModulated Neutrons Fluxes with Sealed Off Neutron Tubes" by C. W. Elengaand O. Reifenscheweiler, published in the Proceedings of the Symposiumon Pulsed Neutron Research, Vol. II, pages 609-622, 10th-14th of May,1965, published by the International Atomic Energy Agency of Vienna,Austria. This paper describes in detail techniques which may be utilizedto provide a smoothly modulated neutron flux from a tube of thedeuterium-tritium accelerator type as a function of time. Other waveshapes than purely harmonic or sinusoidal modulation may be producedalso using these techniques.

For a clearer understanding of the present invention it will be helpfulto consider first an explanation of its underlying principles. In thisexplanation certain simplifying assumptions are made for ease ofanalysis. For example, it is assumed that a spatially distributed sourceof thermal neutrons (or a thermal neutron cloud) is created in the earthformations by the action of the high energy neutron source. Also, it isassumed that the slowing down time T_(AS) for the fast neutrons producedby the deuterium-tritium source is considerably shorter than the thermalneutron decay time T_(a). This assumption is valid, for example, in highporosity fluid saturated sands such as those of interest in oil welllogging.

In general in thermal neutron lifetime well logging it is assumed (andalso generally experienced) that the thermal neutron population asmeasured by a detector in the well bore decays with time after a neutronburst or pulse according to the following mathematical expression:

    n(t) = Ae.sup.-.sup.α.sup.t + Be.sup.-.sup.β.sup.t (3)

In Equation (3), n(t) is the neutron population as measured by thedetector, Ae⁻.sup.α^(t) is the neutron population of the formationcomponent as measured by the detector, and Be⁻.sup.β^(t) is the neutronpopulation of the borehole environment component as measured by thedetector. The constants A and B are the initial thermal neutrondensities at time t = O in the earth formation (A) and the borehole (B)respectively. The constants α and β are the thermal neutron decayconstants of the earth formation (α) and the borehole (β), respectively.

In making measurements of the thermal neutron population as a functionof time it will be recognized by those skilled in the art that thedetector 23 of FIG. 1 could be either a thermal neutron detector (suchas a helium 3 or boron trifluoride detector) or could be gamma raydetector such as a sodium or cesium iodide thallium activated crystal.If a thermal neutron detector, such as the helium 3 or boron trifluoridedetectors are used, then the thermal neutrons are measured directly. Ifa sodium iodide thallium activated crystal detector is used, then gammarays resulting from the capture of thermal neutrons are detected. Thisprovides an indirect, but proportional, measurement of the number ofthermal neutrons present. In either event the thermal neutron populationmay be determined as a function of time by counting the number ofelectrical pulses produced by the detector.

Equation (3) may be Fourier transformed from the time domain into thefrequency domain and may be rewritten in the frequency domain as:##EQU1## In Equation (4), N(ω) is the neutron population in thefrequency (ω) domain. The other symbols A, B, α and β are as previouslydefined.

Irradiating the earth formation and the borehole environment with aphase coherent souce of fast neutrons which is modulated harmonically atan angular frequency (ω), the resulting cloud of thermal neutrons(and/or capture gamma rays as measured by a thermal neutron or gamma raydetector) is also phase coherent and harmonically modulated, but with aphase lag φ relative to the neutron source. The phase lag φ is afunction of the excitation frequency ω, the decay constants α and β ofthe earth formation and the borehole environment, respectively. It maybe shown that the tangent of the phase angle is given by the followingexpression: ##EQU2## where the symbols A, B, α, β, ω and φ are aspreviously defined.

Equation (5) may be simplified by making the substitutions B/A = R, andTan φ/ω = X resulting in: ##EQU3##

Equation (6) contains the three unknown parameters required to determinethe earth formation thermal neutron decay constant α (the reciprocal ofthe thermal neutron decay time), the borehole thermal decay constant β,and the ratio of amplitudes of formation and borehole thermal neutroncomponents R. The variable α is the most desirable one of theseparameters to obtain knowledge of. It is an indication (i.e., thereciprocal) of the formation thermal neutron decay time which, as hasbeen previously discussed, is very important in determining the possiblehydrocarbon content and water saturation of earth formations in thevicinity of a well bore. Since Equation (6) contains three unknowns R,α, and β, at least three independent equations are needed to determinethese three unknowns. The three independent equations for determiningthe three unknowns R, α, and β may be obtained by independentlymeasuring the phase shift at three different modulation frequencies ω₁,ω₂ and ω₃. Assuming that relative phase angle measurements are made atthree different modulation frequencies ω₁, ω₂ and ω₃, three equations ofthe form of Equation (6) are obtained. Combining two of these equations,the unknown variable R may be eliminated resulting in: ##EQU4## where X₁= Tan φ₁ /ω₁ and X₂ = Tan φ₂ /ω₂.

Equation (7) may be rewritten in the following form as a cubic equationin the unknown α (earth formation thermal neutron decay constant).

    a.sub.3 α.sup.3 + a.sub.2 α.sup.2 + a.sub.1 α + a.sub.o = 0                                                         (8)

In Equation (8) the coefficients a_(i) are functions of β, X₁, X₂, ω₁and ω₂ which are given by the following set of equations:

    a.sub.3 = β.sup.2 (x.sub.2 - x.sub.1)+β(ω.sub.2.sup.2 -ω.sub.1.sup.2)x.sub.1 x.sub.2 +(ω.sub.2.sup.2 x.sub.2 -ω.sub.1.sup.2 x.sub.1)

    a.sub.2 = β.sup.3 (x.sub.1 -x.sub.2)+β(ω.sub.2.sup.2 x.sub.1 -ω.sub.1.sup.2 x.sub.2)+(ω.sub.2.sup.2 -ω.sub.1.sup.2)

    a.sub.1 = β.sup.3 (ω.sub.1.sup.2 -ω.sub.2.sup.2)x.sub.1 x.sub.2 +β.sup.2 (ω.sub.1.sup.2 x.sub.2 -ω.sub.2.sup.2 x.sub.1)+(x.sub.2 -x.sub.1)ω.sub.1.sup.2 ω.sub.2.sup.2

    a.sub.o = β.sup.3 (ω.sub.1.sup.2 x.sub.1 -ω.sub.2.sup.2 x.sub.2)+β.sup.2 (ω.sub.1.sup.2 -ω.sub.2.sup.2)+β(x.sub.1 -x.sub.2)ω.sub.1.sup.2 ω.sub.2.sup.2

Similarly measurements may be made at frequencies ω₁ and ω₃ and combinedin Equation (7) yields:

    b.sub.3 α.sup.3 + b.sub.2 α.sup.2 + b.sub.1 α + b.sub.o = 0                                                         (10)

Here the b_(i) are functions of β, X₁, x₃, ω₁, and ω₃ similar to theexpressions given in Equations (9) but substituting X₃ for X₂, and ω₃for ω₂ in Equations (9). Equation (10) may also be written in anotherform as Equation (11):

    c.sub.3 β.sup.3 + c.sub.2 β.sup.2 + c.sub.1 β + c.sub.o = 0 (11)

where the c_(i) are functions of α, X₁, X₃, ω₁ and ω₃. Equations (8) and(11) represent two independent equations with β and α as the unknowns.These equations may be solvent for α and β, the quantities of interest,by using graphical techniques to be described.

The three different frequencies of intensity modulation of the neutronsource chosen for use in the present invention are 400 Hertz, 2000Hertz, and 4000 Hertz. It will be appreciated by those skilled in theart that frequencies other than these may be used if desired withoutcompromising the inventive concepts. However, these frequencies aresuitable for the purposes of this description. Referring now to FIG. 5,a family of curves is shown which plots -Tanφ₄₀₀₀ versus -Tanφ₄₀₀₀ forvarious values of R and with β = 50 microseconds (here β = 1/β). Thesevalues were calculated from Equation (5) using neutron source modulationfrequencies (f = ω/2π) 400 and 4000 Hertz, respectively. FIG. 5a is anexpansion of the region of FIG. 5 near the origin in which β is greaterthan or equal to α (here α = 1/α1/α).

FIG. 4 similarly is a second family of curves where -Tanφ₄₀₀ is plottedagainst -Tanφ₂₀₀₀ for various values of R and α with β = 50microseconds. Again, these values were computed from Equation (5) at therespective frequencies. FIG. 5a is an expansion of the curves of FIG. 5in the vicinity near the origin where β is greater than or equal to α.

If we assume for the present that the borehole has β = 50 microseconds,then by measuring -Tanφ at two frequencies the α (true) (true decay timeof the formation) may be determined by comparing the measurements ofthese values of -Tanφ at the two frequencies and using either thegraphical representation of FIGS. 4 or 6 for this purpose. Similarly, ifthe borehole component decay time β is known, then figures similar toeither FIGS. 4 or 5 could be used to measure this α (true) of theformation by using these graphical representations at whichever pair offrequencies is desired (i.e., 400-2000 or 400-4000).

However, in field operations β is in general not known and can even varywithin a given well depending upon the condition of the borehole fluid,the borehole diameter, the cement thickness or casing size. In this morerealistic situation it is necessary to utilize three frequencymeasurements. In this case both α (true) and β (true) may be determinedin the following manner. First, the charts of FIGS. 4 and 5 are utilizedto determine an apparent α, from each of the two pairs of differentfrequency phase angle determinations (i.e., the α apparent from the400-2000 Hertz combination, and the α apparent with the 400-4000 Hertzmodulation frequencies). Then the spine and ribs plot of FIG. 6 isutilized. In FIG. 6 the value of α apparent at each of these frequencycombinations is plotted. The α (true) is then determined from thelocation of the point on the graphical representation of FIG. 6 whichprovides a unique solution for α (true) regardless of the value of R andβ. Once the α(true) value is determined in this fashion, β may beobtained by substituting the α (true) value back into Equation (7) andsolving this equation for β.

The use of this graphical technique for determining α and β can be morereadily understood by an illustration of a hypothetical example. Assumethat the following values of the phase shift at the three frequencieshave been measured -Tan₄₀₀ = 1.565, -Tan₄₀₀₀ = 1.712, and -Tan₄₀₀₀ =1.410. These data points have been plotted in FIGS. 4, 5 and 6 asillustrated (labelled "EXAMPLE" therein). It may be seen that thesevalues for the phase shift angles indicate α (true) value of 275microseconds. This value is substituted in Equation (7) yielding β = 50microseconds. The value of α (true) may be checked by computing -Tan₄₀₀₀and -Tan₂₀₀₀ using Equation (5) with β = 50 microseconds, and setting R= 1.6. Comparing the calculated -Tanφ values therefrom with thecorresponding hypothetical values which were given in the example showsthat the above-described measurement procedure is self-consistent andyields a unique solution for the α (true) of the formation.

While the above description of the solution of the foregoing equationsfor the true formation decay time, α (true) have been expressed in termsof graphical solutions performed by use of the graphs of FIGS. 4, 5 and6, it will be appreciated by those skilled in the art that thesegraphical representations could be utilized within a properly programmeddigital computer located either at the well site in which themeasurements are made, or in a remote location, if desired. A smallgeneral purpose digital computer such as the model PDP-11 made by theDigital Equipment Corporation of Cambridge, Massachusetts could besuitable for this purpose. The graphical representations correspondingto FIGS. 4, 5, and 6 may be entered in the memory of such a digitalcomputer in the form of tables. Appropriate interpolation techniques maybe utilized to reach the combination of graphical solutions justdescribed with respect to FIGS. 4, 5 and 6. Thus, it is seen if a welllogging tool which can measure the values of the tangent of the phaseshift φ at each of three chosen frequencies of intensity modulation ofthe neutron source is provided, that these measurements of the phaseangle tangent values may then be appropriately combined to derive thetrue formation and borehole thermal neutron decay time parameters ofinterest. It is, of course, well known in the art that once the trueformation and borehole thermal neutron decay time parameters are known,then appropriate techniques which are known in the art may be applied toderive the formation water saturation (and hence the oil saturation ofthe formation, providing the porosity is known from another source).

Referring now to FIGS. 2 and 3 the well logging system shownsystematically in FIG. 1 is illustrated in more detail. Consideringfirst the circuitry of the downhole portion of the system in FIG. 2, itwill be seen that a gamma ray detector comprising a thallium dopedsodium iodide crystal 101 is optically coupled to a photomultiplier tube102 which produces electrical pulses proportional in height to theenergy of the gamma ray impinging upon the sodium iodide crystal 101.The electrical pulse signals from the photomultiplier tube 102 areamplified by a preamplifier 103 and supplied therefrom to a pulse heightdiscriminator 104. Discriminator 104 is used to discriminate againstrelatively low energy background gamma radiation and has an adjustablediscriminator level as indicated by potentiometer 105. This level isusually set at about 0.5 MEV, so that gamma rays resulting fromnaturally occurring background radiation may be discriminated against.The output data pulses from the pulse height discriminator 104 aresupplied via line 106 to three AND gates 107, 108, and 109 whoseconditioning will be described in more detail subsequently.

The transmission of the data pulses to the surface is controlled insynchronization with the operation of the neutron generator tube 110 ofFIG. 2. A 16 kilohertz oscillator driver 111 is utilized to generatetiming pulses. The 16 kilohertz output pulses from the oscillator 111are supplied on a line 112 as inputs to a divide by 16,000 dividercircuit 113, to a divide by 4 divider circuit 114, to a divide by 8divider circuit 115, and to a divide by 40 divider circuit 116. As thefrequency of the 16 kilohertz oscillator is divided by 16,000 in thedivider circuit 113, output pulses are produced by the divider 113 whichoccur once each second during the operation of the system. The once persecond output pulses are provided from divider 113 on line 117 and areused to trigger (via an amplifier 118) the reset of divider circuits114, 115, and 116 once each second.

The once per second output pulses from the divider 113 are also suppliedby to a divide by 3 one shot multivibrator 158 which has multipleoutputs comprising lines 119, 120, and 121. Divide by 3 one shot circuit158 functions, upon the receipt of a pulse on its input line 117, toproduce a voltage level output on line 119 for a one second durationbeginning upon receipt of the first such pulse on its input line 117.Upon receipt of its second input pulse on line 117 (at the end of onesecond of operation) the output voltage level is removed from line 119and applied to line 120. Similarly at the end of the second one secondof operation, the third input pulse is received on input lead 117,divide by 3 one shot 158 produces an output voltage level of one secondduration on line 121. Upon receipt of the fourth one second input pulseon line 117 the output voltage level is removed from line 121 andrestored to line 119. Thus, the divide by 3 one shot 158 providessuccessive conditioning voltage levels on output lines 119, 120, and 121which are used to condition successively three AND gates 122, 123, and124.

Considering now the generation of a 400 Hertz sine wave modulation whichis to be applied to the neutron generator 110, this is accomplished inthe following manner. Input pulses on line 112 to divide by 40multivibrator 116, which it will be recalled is reset at the beginningof each second by output pulses from divide by 16,000 circuit 113,produces a 400 Hertz square wave output on output line 125. This signalis supplied as one input to AND gate 122, during the first second ofoperation of a three second cycle of operation of the apparatus. The ANDgate 122 is conditioned to pass to 400 Hertz square wave pulses suppliedon input line 125 during this initial second of the three secondoperational cycle. The 400 Hertz square wave pulses passed by the ANDgate 122 are thus supplied during this initial second of operation to a400 Hertz band pass filter 126 which shapes the square wave pulses intoa sine wave shape by the action of the tuned circuitry containedtherein. Thus the output of the 400 Hertz band pass filter 126 comprisesa 400 Hertz sine wave which is supplied as input to an amplifier 127 andamplified to a more usable signal level. This signal is supplied asinput to a driver amplifier 128 which is coupled to the neutrongenerator tube ion source and thus applied as a sine wave intensitymodulation to the neutron flux output of the generator tube.

The AND gates 123 and 124 are similarly conditioned during the secondand third seconds of a three second operational cycle of the apparatus.The two kilohertz square wave output of divide by 8 circuit 115 issupplied on input line 129 to AND gate 123. The four kilohertz squarewave output pulses from divide by 4 circuit 114 are supplied on inputline 130 to AND gate 124. Outputs from AND gates 123 and 124 aresimilarly shaped by 2000 Hertz band pass filter 131 and 4000 Hertz bandpass filter 132 to provide sine wave output wave shapes to amplifiers133 and 134 respectively during the second and third seconds of theoperational cycle of the circuit. In this manner, an intensity modulatedneutron output at 400 Hertz for the first second of operation, 2000Hertz for the second second of operation and 4000 Hertz for the thirdsecond of operation are provided. This cycle is depcited graphically inFIG. 8 of the drawings.

Now concerning the synchronization of the transmission of the detectedgamma rays during the different intervals of neutron modulation fortransmission to the surface, it will be observed that the outputconditioning signals from divide by 3 one shot 158 are supplied on lines119, 120 and 121 to AND gates 107, 108, 109, 135, 136 and 137. It willbe recalled that data pulses from the sodium iodide/photomultiplierdetector are continuously coupled to one input of AND gates 107, 108 and109. Similarly, 16 kilohertz clock pulses are supplied via pulse shapercircuit 138 (which may be of conventional design) to the opposite inputsof AND gates 135, 136, and 137. During the first second of operation ofa three second operating cycle the AND gate 109 and the AND gate 137 areconditioned for operation while the remaining AND gates 107, 108, 135and 136 are in a blocked condition. Thus, during the first second ofoperation data pulses from the detector are passed by AND gate 109 andthe AND gate 137 passes the 16 kilohertz clock pulses. These pulses aresupplied with the clock pulses being of one polarity and the data pulsesbeing of the opposite polarity due to the action of inverters 140, 141and 142 in reversing the polarity of the clock pulses. The data pulsesand the clock pulses are summed at mixing point 143 during the firstsecond of operation. The mixed clock and data pulses of oppositepolarity are then linearly amplified by a predetermined scale factor byamplifier 144 during this first second of operation of a three secondcycle of the neutron source which is being modulated at a 400 Hertzfrequency, the data pulses detected during this portion of the cycle andthe clock pulses from the 16 kilohertz clock oscillator 111 are appliedto a pulse transmission circuit 145 for transmission to the surface.Similarly, during the second second of operation data pulses from thedetector system are supplied together with the 16 kilohertz clock pulsesvia a second scale factor amplifier 146 to the pulse transmissioncircuit 145 for transmission to the surface. Finally, during the thirdsecond of the three second operating cycle, mixed data pulses and clockpulses are supplied to the pulse transmission circuit 145 via a thirdscale factor amplifier 147. The amplification levels of the scale factoramplifiers 144, 146, and 147 are set substantially apart so that at thesurface, the signals representing the clock pulses and the data pulsesoccurring during the three different time gating intervals may bediscriminated against each other by the use of a pulse heightdiscriminator as will be described. That is to say, the scale factoramplifier 144 may have a gain factor of 10 while the scale factoramplifier 146 may have a gain factor of 20 and the scale factoramplifier 147 may have a gain factor of 30. Thus, during the threedifferent seconds of the operational cycle, signal levels transmitted tothe pulse transmission circuit 145 have substantially different voltagelevels prior to their introduction to the cable for transmission to thesurface. This transmission scheme is depicted graphically in FIG. 7.

Output signals from the pulse transmission circuit 145 are capacitivelycoupled via a capacitor 153 to the center conductor 152 of the welllogging cable. High voltage D.C. for the operation of thephoto-multiplier tube 102 is also provided on the center conductor ofthis cable from surface power supplies. The B+ voltage for operation ofthe preamplifier 103 is supplied from a surface power supply on theshield 151 of this cable. Thus a signal wave form as illustrated in FIG.7 is applied to the cable during the operation of the downholeequipment. In the illustration of FIG. 7 the first second of operation(illustrated at 201) is characterized by the transmission of negativesharp spike synchronization and data pulses occurring at a randominterval following each of the 16 kilohertz synchronization pulses.Similarly, during the second second of operation (202 of FIG. 7) thenegative sharp spike synchronization pulses occur, but with a largervoltage amplitude than during the first second of operation. Therandomly occurring spike data pulses again have a positive voltage andfollow each sync pulse. Finally, during the third second of operation(as illustrated at 203 in FIG. 7) the negative 16 kilohertz clock pulsesoccur at regularly spaced intervals. Each sync pulse is followed byplurality of randomly occurring positive voltage level pulsescorresponding to gamma ray counts made during the intervals between the16 kilohertz data pulses.

This information is transmitted to the surface where it is interpretedin terms of the tangents of the phase angles occurring at each of thethree different modulation frequencies by the data processing circuitryshown in FIG. 3. Prior to the consideration of the operation of thecircuitry of FIG. 3, however, it will be appropriate to consider how thetangent of the phase angle is determined. Referring to the drawing ofFIG. 9 a single cycle of the intensity modulated neutron cloud at theneutron source (solid curve) and at the neutron detector (dashed curve)is illustrated schematically. The straight line (labelled N(avg))represents the average neutron population generated. It will be observedthat a phase shift φ between the neutron population at the source and atthe detector due to the neutron lifetime (or thermal neutron decay time)of the borehole and formation materials, exists as previously discussed.If the neutron source modulation cycle begins at t_(o) as shown, and isdivided into four quadrants as a function of time, these quadrants willend at t₁, t₂, t₃, and t₄ and will each be of a duration t₁ - t₀ whichis dependent on the modulation frequency ω. If the counts of capturegamma rays occurring at the detector during each of the four quadrantsof the modulation cycle are labelled C₁, C₂, C₃, and C₄, then it may beshown that the tangent of the phase angle φ is given by: ##EQU5##

Thus, it is possible by determining the counts of gamma rays occurringin each of the four quadrants of a cycle of modulation of the fastneutron source, to derive the relative phase angle φ of the phasecoherent neutron cloud at the detector with respect to the modulatedcloud of thermalized neutrons produced by the fast neutron source. Theonly approximation used in making this derivation of the tangent of thephase angle is that the slowing down time of the fast neutrons is shortwith respect to the thermal neutron decay time (or neutron lifetime) ofthe earth formations in the vicinity of the well borehole. This isusually an excellent approximation.

Referring now to FIG. 3, the surface data processing signal equipment isillustrated in block diagram form. The well logging cable 150 centerconductor 304 is provided with high voltage for the operation of thephotomultiplier tube (102 of FIG. 2) by high voltage power supply 305which is resistively coupled thereto by resistor 303. The B+ voltage forthe operation of associated downhole circuits is coupled from B+ powersupply 383 to the inner coaxial shield of the logging cable 150 in asimilar manner. The alternating current data signals produced by thedownhole equipment are extracted via a coupling capacitor 302 andsupplied as input to a pulse separator 306. It will be recalled that the16 kilohertz synchronization pulses are provided as sharp peakedpositive going voltages on this cable while the data pulses are providedas randomly occurring negative sharp peaked pulses. The pulse separator306 functions to provide output pulses on two lines with the 16kilohertz synchronization pulses occurring on output line 307 and therandomly occurring data pulses on output line 308.

It will also be recalled that the three different frequency portions ofmodulation of the downhole neutron generator were characterized by thethree different levels of voltage amplifications which are applied toboth the synchronization pulses and clock pulses at the downhole tool.This results in three different voltage amplitude components whichcorrespond to the three different frequency modes of operation beingsupplied to the surface equipment. Thus, the output synchronizationpulses on line 307 occur at three different characteristic voltagelevels when output from the separator 306. These synchronization pulsesare input to three voltage level discriminators 309, 310, and 311.Voltage level discriminators 309, 310 and 311 separate the clock pulseswhich occur during each period of one second duration of the differentfrequency modulations applied to the downhole neutron source on thebasis of their amplitudes. Voltage level discriminator 309 thus permitsonly clock pulses occurring during the 400 Hertz modulation to be outputtherefrom on line 312. Similarly, 2000 Hertz voltage discriminator 310only permits synchronization or clock pulses to be output therefrom online 313, while 4000 Hertz voltage discriminator 311 only permits clockpulses to be output therefrom on line 314 during the 4000 Hertzmodulation.

Voltage level discriminators 309, 310, and 311 thus provide clock orsync pulse output on their respective output lines 312, 313, and 314only while their respective modulation frequency periods are occurringin the downhole tool. Considering now the operation of the portion ofthe circuitry which concerns the 400 Hertz intensity modulation of thedownhole neutron source, the occurrence of the clock or sync pulses online 312 (which is permitted to enter at the beginning of the 400 Hertzmodulation) is used to set (after a 2.5 millisecond delay provided by adelay divide by 40 circuit 380) a flip-flop 315 and to immediately reseta second flip-flop 316 in the 4000 Hertz modulation portion of thecircuit. Similarly, the occurrence of the clock pulses on output line313 is used to set (after a 2.5 millisecond delay provided by delaydivide by 40 circuit 381) a flip-flop 317 and to immediately reset theflip-flop 315. This action causes the three channels of the circuit(corresponding to the three different frequencies of modulation) to bealternately and singly activated for counting purposes as the voltagelevel on the output lines of flip-flops 315, 316 and 317 are used tocondition a plurality of AND gates and will be described subsequently.The 2.5 millisecond delays provided by delay circuits 380, 381 and 382are included to allow any phase shift effects of thermal neutrons fromthe just completed different modulation frequency portion of theoperating cycle to die away before beginning to make the phase shiftdetermination at the new operating frequency. A few counts will be lostin this manner (only about 0.25%) but increased accuracy will result.The 2.5 millisecond delay is one full period of 400 Hertz modulation,five periods of 2000 Hertz modulation and 10 periods of 4000 Hertzmodulation. These times far exceed the amount of phase shift to bemeasured and will allow effects from the previous modulation cycle todissipate prior to the beginning of measurement.

Returning now to the consideration of the 400 Hertz modulationprocessing circuitry, the clock pulses occurring on line 312 aresupplied as input to a countershift register 318 during a 400 Hertzmodulation of the downhole neutron source. One complete cycle ofmodulation of the source occurs each one four-hundredth of a second(2500 microseconds). The clock pulses at 16 kilohertz appear once each62.5 microseconds. Thus, upon the occurrence of each ten clock pulsessupplied to the counter shift register 318, one quadrant of one cycle ofmodulation of the downhole neutron source occurs. Countershift register318 provides an output voltage level on line 319 during the firstquadrant of one cycle of modulation; it provides an output voltage levelon line 320 during the second quadrant of such cycle of modulation; itprovides an output on line 321 during the third quadrant of the cycle ofmodulation; and it provides an output voltage level on line 322 duringthe fourth quadrant of the cycle of modulation. These output voltagelevels provided successively on lines 319, 320, 321 and 322 are alsoused in conditioning the plurality of AND gates associated with thiscircuitry. It should be mentioned here that the counter-shift register318 is also reset by the appearance of the first clock pulse appearingon output line 313 from the 2000 Hertz voltage level discriminator 310,so that upon completion of one second of the 400 Hertz modulation, thefirst clock pulse appearing on output line 313 clears the counter-shiftregister 318 circuit and enables it to start counting anew upon theinitialization of the next 400 Hertz modulation portion of theoperational cycle of the downhole equipment.

The AND gates 323, 324, 325 and 326 are associated with the 400 Hertzchannel of the circuit of FIG. 3. These multiple input AND gates requirethe presence of a voltage level on each of their three input leadsbefore they will produce a pulse. One input lead of each of AND gates323, 324, 325 and 326 is connected to the gamma ray count data inputline 308 from the pulse separator circuit 306. A second input lead ofeach of these AND gates is connected to the conditioning flip-flop 315described previously, and the third input lead of each of these multipleinput AND gates is connected to one of the output lines of counter-shiftregister 318, which corresponds to one of the quadrants of a cycle ofmodulation of the downhole neutron source. Thus, during the firstquadrant of operation of a 400 Hertz modulation cycle in the downholetool (following the 2.5 millisecond delay), data pulses are permittedonly through AND gate 323. Similarly, during the second quadrant of a400 Hertz modulation cycle the data pulses are only permitted throughAND gate 324 and, similarly, AND gates 325 and 326 permit the passage ofdata pulses occurring only during the third quadrant and fourth quadrantrespectively of a cycle of 400 Hertz modulation. Four OR gates 327, 328,329, and 330 together with a pair of up/down counters 331 and 332 areused to form the expressions of the numerator and denominator of thefraction of Equation (12) in the following manner.

C₁ counts occurring during the first quadrant of the modulation cyclefrom AND gate 323 are supplied as inputs to OR gates 327 and 329.Similarly C₂, second quadrant, outputs from AND gate 324 are supplied asinputs to OR gates 328 and 329. C₃ counts occurring during the thirdquadrant of the 400 Hertz modulation cycles are supplied as inputs to ORgates 328 and 330. Finally, during the fourth quadrant of modulation, C₄counts are supplied to OR gates 330 and 327. The numerator of thefraction of Equation (12) is formed in up/down counter 331 while thedenominator of this expression is formed in up/down counter 332.Observing the numerator of the fraction of the expression of Equation(12) it will be noticed that the expression may be formed by counting upduring quadrants 1 and 4 in counter 331 and counting down (subtracting)during quadrants 2 and 3 in the same counter. Thus, the C₂ and C₃ countsare supplied to the down count input terminal of up/down counter 331while the C₁ and C₄ counts are supplied to the up/count input terminalin this counter. Thus, at the end of each complete cycle of 400 Hertzmodulation of the neutron source the up/down counter 331 contains adigital number representative of the numerator of the fraction of theexpression of Equation (12).

Similarly, the expression of the denominator of the fraction of Equation(12) is formed during each cycle of modulation in the up/down counter332. Thus during the one-second 400 Hertz modulation period of a givenoperational sequence in the downhole tool, up/down counters 331 and 332contain the expressions representing the numerator and denominator ofthe fraction of Equation (12).

Now concerning the extraction of these digital numbers contained incounters 331 and 332, the plurality of one shot and delay one shotmultivibrators 333 - 341 is used for this purpose. Upon the occurrenceof an output voltage level from the flip-flop 315, a voltage is appliedon line 342 to a delay one shot 336 and to a transfer one shot 333. Theoccurrence of a voltage pulse on one shot 333 causes and X' transfervoltage to be output from the one shot which is supplied to quad latchcircuits 343 and 344. This causes the quad latch circuits 343 and 344 toextract the digital number contained in the up/down counters 331 and 332at that time. The presentation of this voltage level on line 342 to thedelay one shot 336 causes the occurrence (at a time one second later) ofan output voltage from the delay one shot 336 which is input to a oneshot multivibrator 339. Multivibrator 339, in turn, provides an output Xvoltage for resetting the up/down counters 331, 332.

The result of the X transfer one shot voltage levels and the X' resetvoltage levels is to cause the output of up/down counters 331 and 332 tobe extracted at the end of each one second period of 400 Hertzmodulation of the neutron source and the up/down counters forming thenumerator and denominator of the fraction of Equation (12) to be resetfollowing each one second period of operation at the 400 Hertz frequencyof modulation in the downhole tool. In a similar manner, one shots 334and 337 are responsive to input voltages on lines 350 and 351corresponding to the occurrence of output clock pulses from flip-flops317 and 316, respectively and cause the extraction into quad latchcircuits 352, 353, 354 and 355 of the numerator and denominator signalsaccumulated in a similar manner in the up/down counters 356, 357, 358and 359 of the circuits corresponding to the 2000 Hertz and 4000 Hertzmodulation portions of the operative cycle of the downhole tool.

Reutrning to consideration of the 400 Hertz modulation channel, thedigital numbers occurring at the end of the one second periods ofmodulation in the quad latches 343 and 344 are then applied to digitalto analog converter circuits 360 and 361 which convert these into analogvoltage levels are applied to an analog ratio circuit 362 which providesan output signal equal to the expression -Tanφ₄₀₀ (or tangent phaseangle at 400 Hertz). In a similar manner, digital to analog convertercircuits 363, 364, 365 and 366 provide output voltage levelscorresponding to the numerator and denominator of the fractions of theexpression of Equation (12) for the other two operating frequencies of2000 Hertz and 4000 Hertz modulation. These signals are then supplied asinputs to analog ratio circuits 367 and 368 to provide output signalsrepresentative of the tangent of the phase angle occurring at each ofthese frequencies of modulation. Thus, analog ratio circuits 362, 367and 368 provide analog output voltages representative of the tangent ofthe phase angle occurring at each of the operative modulationfrequencies of the downhole neutron source. These quantities may besupplied to recording channels in the recorder 18 of FIG. 1 forrecording as a function of the borehole depth. Alternatively they can besupplied to digital computing apparatus, as previously discussed, forcomputing directly the borehole and formation components of the thermalneutron lifetime in the manner previously described.

While only the 400 Hertz channel of the data processing circuit of FIG.3 has been described in detail, it will be appreciated that the 2000 and4000 Hertz channels function in an analogous manner. The onlydifferences between these channels occurs in the timing of the foursequential outputs of counter shift registers 369 and 370 which occur attimes appropriate to delineate quadrant counts at each of thesemodulation frequencies appropriately.

The foregoing descriptions may make other alternative embodiments inaccordance with the concepts of the present invention apparent to thoseskilled in the art. It is therefore the aim of the appended claims tocover all such changes and modifications as fall within the true spiritand scope of the invention.

I claim:
 1. A method for determining the thermal neutron decay time of earth formations in the vicinity of a well borehole comprising the steps of:continuously irradiating the earth formations in the vicinity of a well borehole with a phase coherent harmonically intensity modulated cloud of fast neutrons having at least three different frequency modulation components; detecting as a function of time the intensity modulated thermal neutron population resulting from said irradiation at said at least three different modulation frequencies; determining from said detected thermal neutron populations measured as a function of time, the relative phase shift between said fast neutron cloud and said thermal neutron population at said at least three different modulation frequencies; and combining said three different modulation frequency relative phase shift measurements according to a predetermined relationship to derive an indication of the thermal neutron decay time of the earth formations in the vicinity of a well borehole.
 2. The method of claim 1 and further including the step of:combining said three different modulation frequency relative phase shift measurements according to a predetermined relationship to derive an indication of the thermal neutron decay time of the borehole fluid in a well borehole.
 3. The method of claim 1 and further including the step of performing said method steps along a borehole as a function of depth and recording said indications of the thermal neutron decay times so measured as a function of borehole depth.
 4. The method of claim 2 and further including the step of performing said method steps along a borehole as a function of depth and recording said indications of earth formation thermal neutron decay time and borehole fluid thermal neutron decay time as a function of borehole depth.
 5. The method of claim 1 wherein the step of determining the relative phase shift between said fast neutron cloud and said thermal neutron population at said at least three different modulation frequencies is performed by separating the modulation cycle at each of said at least three different modulation frequencies into at least four equal time intervals corresponding to quadrant cycle intervals at the respective modulation frequency and measuring the thermal neutron population in such quadrant cycle intervals and combining such measurements in a predetermined manner to derive an indication of said relative phase shift.
 6. The method of claim 5 wherein said measurements of the thermal neutron population in said quadrant cycle intervals are combined to derive a measure of the tangent of the relative phase shift between said fast neutron generation and said thermal neutron population at each such modulation frequency.
 7. The method of claim 6 wherein said quadrant cycle measurements are combined according to the relation ##EQU6## where φ is the relative phase shift angle and C₁, C₂, C₃, and C₄ are count signals representative of the thermal neutron population measurements in the first, second, third and fourth quadrants of a modulation cycle respectively.
 8. A method for logging earth formations traversed by a fluid filled well borehole to develop a measurement of thermal neutron interaction characteristics of such formations comprising the steps of:moving a well logging sonde sized and adapted for passage through a well borehole past earth formations penetrated by a well borehole; continuously generating fast neutrons in said sonde and bombarding the surrounding media with such neutrons to develop a thermal population in the borehole and surrounding media; harmonically modulating the intensity of said fast neutrons at, at least three different modulation frequencies; detecting the thermal neutron population in the vicinity of said sonde as a function of time and developing signals representative thereof; and combining said representative signals according to predetermined relationships to derive an indication of the thermal neutron decay time of the earth formations in the vicinity of the borehole.
 9. The method of claim 8 and further including the step of combining said representative signals according to predetermined relationships to simultaneously derive an indication of the thermal neutron decay time of the borehole fluid.
 10. The method of claim 9 and further including the step of recording said indications of the thermal neutron decay time characteristics of the earth formations and the borehole fluid as a function of the borehole depth of said sonde.
 11. The method of claim 8 wherein the step of detecting the thermal neutron population as a function of time is performed by detecting said thermal neutron population during quadrant intervals of a cycle of modulation at each of said at least three different intensity modulation frequencies and generating signals representative thereof.
 12. The method of claim 11 and further including the step of combining said representative quadrant signals in a predetermined manner to derive indications of the relative phase shift between the intensity of the fast neutrons and the thermal neutron population at each of said different frequencies of intensity modulation.
 13. The method of claim 12 wherein said representative quadrant signals are combined to derive an indication of the tangent values of the phase shift angles at each of said different frequencies of intensity modulation.
 14. The method of claim 8 wherein the step of harmonically modulating the intensity of said fast neutrons at, at least three different modulation frequencies is performed by modulating said intensity at a first frequency for a first time interval followed by immediately modulating said intensity at a second frequency for a second time interval followed by immediately modulating said intensity at a third frequency for a third time interval to form a modulation sequence and repetitively repeating this modulating sequence as said sonde is moved through the borehole.
 15. The method of claim 14 wherein said first, second and third time intervals are of equal duration.
 16. The method of claim 14 wherein said step of detecting the thermal neutron population as a function of time is performed by detecting said thermal neutron population during quadrant cycle intervals of said first frequency during said first time interval, detecting the thermal neutron population during quadrant cycle intervals of said second frequency during said second time interval, and detecting the thermal neutron during quadrant cycle intervals of said third frequency during said third time interval.
 17. The method of claim 16 wherein said first, second and third time intervals are of equal duration.
 18. The method of claim 17 wherein said modulation frequencies are chosen to be 400 Hertz, 2000 Hertz and 4000 Hertz respectively.
 19. The method of claim 16 and further including the step of delaying, for a predetermined duration, at the beginning of each of said first, second and third time intervals the detection of the thermal neutron population to allow phase shift effects from prior modulation frequency interval to substantially die away and then resuming said detection for the remainder of said time interval.
 20. The method of claim 19 wherein said predetermined delay duration comprises an integral number of modulation cycles of each of said different modulation frequencies. 